function teacherTestClass2(){
    var tom = new Teacher('Tom' , 21 , 2);
    var expectResult = 'My name is Tom. I am 21 years old. I am a Teacher. I teach Class 2.';
    (expectResult === tom.introduce()) ? console.log('teacherTestClass2 passed!') : console.log('teacherTestClass2 failed!');
}
function teacherTestNoClass(){
    var tom = new Teacher('Tom' , 21 , null);
    var expectResult = 'My name is Tom. I am 21 years old. I am a Teacher. I teach No Class.';
    (expectResult === tom.introduce()) ? console.log('teacherTestNoClass passed!') : console.log('teacherTestNoClass failed!');
}
teacherTestClass2();
teacherTestNoClass();